This invention relates to an ink jet printer having a print head. More particularly, this invention relates to an energization system which alters the power applied to ink jets carried on the print head of an ink jet printer to reduce artifacts in the printed output.
Ink jet printing involves placing a number of tiny ink droplets formed by one or more ink jets onto particular locations on a printing medium (usually paper). The ink droplets solidify (or dry, or freeze) on the printing medium, forming small dots. A substantial number of these small dots, when viewed from some distance away, are perceived as a continuous visual image. Both text and graphic images may be printed with ink jet printing.
The printed image from an ink jet printer is made up of a gridlike pattern of potential dot locations, called picture elements or "pixels". For many documents commonly viewed from 1-20 feet away, the ink jet printing industry today often uses a print resolution of 300 pixels per inch (90,000 pixels per square inch). The print resolution for other applications may vary as needed, so for the example of printing a billboard that is commonly viewed from hundreds of feet away, the pixel sizes used may result in a density on the order of 6 pixels per inch.
Presently there are two primary types of initiators used to form ink jets for ink jet printers. Resistance heating based jets use a small resistor to heat a portion of ink and create a minute bubble within the ink. The bubble immediately bursts to propel a small droplet of ink through a nozzle. Piezoelectric displacement force based jets use a substrate which is electrically vibrated to create a pressure wave which in turn forces a droplet of ink through a jet nozzle. A method of making a piezoelectric initiator for an ink jet is taught in U.S. Pat. No. 5,265,315 to Hoisington et al., which is incorporated herein by reference. There are also ultrasonic ink jet heads and electrostatic ink jet heads available.
Ink jet printers may further be classified as "on demand", for which ink droplets are formed only at the particular pixel locations at which ink is desired to be placed, or as "continuous", for which ink droplets are formed for each pixel location, but droplets in flight for locations at which ink is not desired are deflected away before they contact the medium. In order to distribute an ink droplet in an on demand type ink jet head having piezoelectric transducers for forming the ink droplets, a high voltage pulse is applied to each piezoelectric transducer when its associated ink droplet is demanded. Each such transducer is part of a mechanical ink forcing arrangement comprising, in general, a piezoelectric crystal plate and a metallic diaphragm. When subjected to a high voltage pulse, the piezoelectric transducer is caused to deform against the diaphragm to pressurize ink in an ink chamber in communication with an ink jet nozzle or orifice, whereby the ink droplet is discharged through the nozzle. When the high voltage pulse is removed, the piezoelectric transducer returns to its initial shape so that a negative pressure is produced in the ink chamber and consequently further ink flows into the ink chamber from a supply source.
Additionally, the inks used in different kinds of ink jet printers vary. Some ink jet printers utilize aqueous ink, which are liquid at room temperature and are generally absorbed into the print medium. Others use "hot melt" ink, which is a solid at room temperature but is applied in a heated liquid condition to then effectively freeze onto the medium. The head assembly energization system of the present invention applies equally to all these various types of ink jet printers, but is particularly contemplated for on demand, piezoelectric, hot melt ink jet printing.
Color ink jet printers usually use the three subtractive primary colored inks, (magenta, cyan and yellow) in addition to black ink. Color blending of these four ink colors is achieved through two mechanisms. First, the ink jet printer may provide ink dots of multiple colors on the same pixel location, thus combining the subtractive effects of these colored inks on light reflected from that pixel. The particular color combination caused by having multiple ink colors at a particular pixel location may be affected by the order of printing the various colored inks.
Second, when viewed at a distance, the eye will perceive blended colors from pure primary colored ink dots provided at adjacent pixel locations. Thus, for instance, a number of exclusively magenta and yellow dots may be provided immediately adjacent to one another in an area of the image, with no pixel location receiving two overlapping inks. Rather than perceiving individual magenta and yellow dots, the eye will instead perceive a blend of the adjacent dot colors to result in the perception of a larger orange dot. In practice, ink jet color printers use both ink blending at particular pixel locations and perception blending across pixel locations to create various colors and shades. In addition, a substantial number of the pixels of the image will go without having a dot of ink placed on them. This allows the perceived visual image to have a proper lightness/darkness value. Through both forms of color blending, ink jet printers using only four colors of ink can visually reproduce full color images.
Ink jet printers generally move a print head containing ink jets back and forth across the printed image while advancing the paper lengthwise in between such passes, or scans, of the print head. To increase the rate of printing, numerous jets per color have been used to create a wider print head "swath", or wider inked surface strip per pass. One prior ink jet color printer utilized a single head having 64 linearly aligned jets. Each jet nozzle was vertically offset one pixel lower than the preceding nozzle. This line of 64 nozzles is divided into four sets of 16 adjacent nozzles, each such set being supplied with one of the colored inks. With this previous 64-jet printer in low quality printing mode, the paper being printed upon was advanced a length equal to 16 pixels after each scan (one quarter of the width of the 64 pixel print stroke), such that each scan of the printer head placed another set of 16 nozzles over the same path across a 16 pixel strip taken by the preceding set. This providing of four scans across a 16 pixel strip was done for each such strip printed on the medium.
Prior art scanning print head configurations, with numerous jets per color each mounted one pixel beneath the previous jet, predicate what is known as a "banding" problem and produces visible artifacts or errors in the printed output. One type of banding occurs if the paper advance is not extremely accurate, such that the paper is advanced slightly more or slightly less than the width of the print swath or stroke (i.e., the vertical extent of the line of jets). Thus, if the paper advances slightly too far a perceptible blank area will occur in the color pattern at the end of each paper advance, between the printed swaths. Conversely, if the paper advance is too short, a perceptible overlap will occur in the color pattern at the beginning of each paper advance, resulting in a darkened region where adjoining swaths overlap.
Other causes can further complicate the banding problem. With some printers, the direction that the print head is traveling for any given scan may affect the order that the different ink colors are laid down on the paper. A different ordering of colors may create a slightly different hue when visually perceived. For instance, if one band is laid down from left to right with magenta over cyan on a significant number of pixels, and the succeeding band is laid down from right to left with cyan over magenta on a significant number of pixels, a slight color difference between the two bands may be visually detectable.
Banding may also be caused in part by the thermal characteristics of the printing scan. In a hot melt ink system, the top of the band may be laid down first, on a relatively cool piece of paper, whereas the middle and bottom of the band may be laid down on a paper heated by previous ink dots. This difference in heating can affect the ink flow characteristics and cause a visually perceptible difference between the top and bottom of the band.
Banding is also caused by misalignment of ink jet heads in ink jet printers having multiple ink jet heads. These alignment problems become aggravated as the number of jets increase, as the spacing between the furthest jets increases during replacement of any other components of the print heads, and as the ink delivery and mechanical placement of print heads becomes more complicated. Alignment of multiple heads is not easily accomplished through mechanical manipulation of the jets, although compensation for some head alignment problems can be accomplished by adjusting the timing of jet firing between different jets. Calibration techniques can be used to determine what adjustment is necessary.
Banding may also be the result of differences in ink droplet size across a swath. For example, one end of the line of jets on the print head may produce slightly larger droplets than the other end, or the center of the line of jets may produce different sized ink droplets than the ends of the line of jets. The different droplet sizes may occur due to voltage drop across the head during firing. If the difference in ink droplet size occurs repeatedly during printing, artifacts between swaths may be visible in the printed output.
Other types of artifacts in the printed output can occur if the print head travel is not entirely uniform. Any vibration or displacement or change in velocity of the print head on a scan across the image may affect the dot placement location. Particularly in cases where the problem occurs regularly at the same location, the printed output will contain discreet artifacts which are perceived by the viewer.
Various methods have been attempted to compensate for these artifact problems. For instance, in U.S. Pat. No. 5,075,689 to Hoisington et al., banding was addressed by altering the arrangement of print jets out of a linear array. Another approach to banding, taught by U.S. Pat. No. 5,239,312 to Merna et al., involves altering the spacing between jets on a print head. Both of these previous methods involve additional manufacturing costs in aligning the ink jets into a non-uniform pattern.
Another approach is "multipass" printing, where the printing medium is advanced at a fractional increment of the vertical swath width, such that two or more jets of the same color pass over a pixel row on subsequent passes. The first jet will generally only print a portion of the dots on that pixel row, with remaining dots on the pixel row printed on subsequent passes. Alternatively or in combination, several of the jets may be deactivated on each pass. With only a portion of the jets actively printing on each pass, additional passes across the image are required for full printing. Multipass printing tends to mask paper advance errors such that they do not appear as discreet artifacts in the print output.
High voltage energization circuits for ink on demand type ink jet heads are known. These known circuits have been located away from the head and are connected to the transducers in the head by routing wires. The routing wires must be of a substantial length to accommodate the head travel across the width of the printing medium for printing. The high voltage energization circuits commonly used in the prior art transfer high voltage signals from the energization circuit to the ink jet head. There are a number of drawbacks to such known high voltage conversion circuits for ink jet heads. Above 42.4 volts peak, or 60 volts DC, power supply circuits fall into the "high voltage" category according to Underwriters Laboratory (UL) standards. UL 1950 Information Technology Equipment, Feb. 26, 1993, Section 1.2.8.4. If a signal is qualified as a high voltage signal, components such as covered connectors, wire shielding, access panel interlocks and the like are required to satisfy UL standards for safe design. These things all add unwanted restraining forces and inertia to the head and so to the mechanical drive requirements for the head assembly which must be capable of rapid movement to achieve high printing rates, and add substantial cost.
Radio frequency emissions are also a concern when routing high voltage signals from an energization circuit to an ink jet head. UL sets requirements for maximum acceptable limits on radio frequency emissions in view of the requirements of the Federal Communications Commission. Emissions higher than the acceptable limits must be reduced by shielding at a substantial cost when routing high voltage signals from an energization circuit to an ink jet head. Shielding adds additional restraining force as well.